The concept of lipids comprises three classes of natural products, which can be isolated from biological membranes: phospholipids, sphingolipids and cholesterol with its derivatives. However, it also comprises synthetically produced materials with similar characteristics. As representatives of these, diacyl glycerols, dialkyl glycerols, 3-amino-1,2-dihydroxypropane esters or ethers and also N,N-dialkylamines are mentioned.
These substances are of technical interest for the preparation of liposomes. One of the uses of these liposomes is as a container for active ingredients in pharmaceutical preparations. For this purpose, an efficient and stable packaging of the cargo, compatibility with body fluids and a controllable and optionally site-specific release of the content are desirable.
It is a disadvantage that it is difficult to combine the two requirements. The tighter and more stable the packaging, the more difficult it is to release the enclosed active ingredient once again. For this reason, liposomes were developed, which change their properties in reaction to external stimuli. Heat-sensitive and pH-sensitive liposomes are known. The pH-sensitive liposomes are of special interest, since this parameter may change under physiological circumstances, such as during the endocytotic absorption of a liposome in cells or during passage through the gastrointestinal tract. According to the state of the art, pH-sensitive liposomes comprise, in particular, cholesterol hemisuccinate (CHEMS).
Cholesterol hemisuccinate is used in admixture with phosphatidyl ethanolamine for the preparation of pH-sensitive liposomes (Tachibana et al. (1998); BBRC 251: 538-544) U.S. Pat. No. 4,891,208). Such liposomes can be endocytized by many cells and in this way are able to transport cargo molecules into the interior of cells, without injuring the integrity of the cellular membrane.
The anionic character of CHEMS is a significant disadvantage. The liposomes, prepared with it, have an overall negative charge and absorbed by cells only with a low efficiency. Therefore, in spite of the transfer mechanism described above, they are hardly suitable for transporting macromolecules into cells.
Cationic liposomes with the highest possible and constant surface charge are used to transport active ingredients into cells (transfection). The overall positive charge of such particles leads to an electrostatic adhesion to cells and, consequently, to an efficient transport into cells. The use of these compounds and of the liposomes, produced therewith is, however, limited to in vitro or ex vitro uses, since such positively charged liposomes form uncontrolled aggregates with serum components.
The limitation to very few pK values, generally to that of the carboxyl group in the cholesterol hemisuccinate (approximately 4.5) is a disadvantage of the pH-sensitive liposomes, which are available according to the state of the art. A further disadvantage of the compounds is the limitation to negative charge carriers. These are not suitable for binding nucleic acids and, frequently also, proteins efficiently.
Cationic liposomes show good bonding of nucleic acids and proteins and are in a position to bring these active ingredients into cells. It is a disadvantage that they cannot be used for in vivo applications.